Satellite system and method of deploying same

ABSTRACT

A satellite communication system has a first deployment of a plurality of satellites deployed in a medium earth orbit and two later deployments of a plurality of satellites deployed in the medium earth orbit. The first deployment is, spaced so that the second deployment may be easily deployed and interleaved into the first deployment. A ground terminal is used for communicating with the satellites in the first and second deployments.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional of pending U.S. patent application Ser. No.10/230,424, filed Aug. 28, 2002 which was a continuation of Ser. No.10/008,675, filed Oct. 23, 2001, now issued as U.S. Pat. No. 6,460,808on Oct. 8, 2002, which was a continuation of Ser. No. 09/783,862 filedon Feb. 13, 2001 now issued as U.S. Pat. No. 6,336,612 on Jan. 8, 2002,which was a divisional of Ser. No. 09/188,440, filed Nov. 9, 1998,issued as U.S. Pat. No. 6,257,526 on Jul. 10, 2001, the entire contentsof all four prior applications being incorporated herein by thisreference.

TECHNICAL FIELD

[0002] The present invention relates to space and communicationssatellites, and more particularly, to a coordinated system for providingsatellite communications using multiple satellites.

BACKGROUND OF THE INVENTION

[0003] Wired terrestrial systems offer communications at high datarates, but only while the user is sitting behind a computer. As soon asthe user goes to a conference room, walks outside an office building,gets into a car, or drives to a park, the connection is lost. Mobility,however, can be supported in one of two ways, namely terrestrial-basedwireless networks or satellite-based communications systems.

[0004] Terrestrial-based wireless networks provide voice or datacommunications between a mobile user and a fixed user or to other mobileusers, as well as communications for modem-equipped computers and othersimilar devices such as mobile facsimile machines. Existing wirelessnetworks have not been optimized for a mix of voice, data, and video,however, despite the trend towards multimedia traffic. Several wirelessand wired standards, such as asynchronous transfer mode (ATM), are beingdesigned to optimize multimedia traffic. Wireless wide area networks(WANs) typically carry voice, whereas wireless local area networks(LANs) typically carry data. Most wireless WAN traffic operates at under19.2 kbps. Wireless LANs that support data rates up to 10 Mbps havebegun to appear, but they are limited in range to tens of meters.

[0005] To provide wireless service, satellite-based communicationssystems have been proposed which would provide world-wide coverage.These proposed systems typically include a constellation of satellitesin one orbit only, such as geostationary earth orbit (GEO) only ornon-geostationary orbits (NGSO). Communications satellites ingeostationary orbit provide coverage in predetermined areas on the earthfrom the equator. Coverage is typically excluded from the oceans so thatsatellite capacity is not wasted on non-populated areas. Communicationssatellites in geostationary orbit, however, provide limited coverage athigher or lower latitudes than the Equator.

[0006] Communications satellites in nongeostationary orbit, such asmedium earth orbit (MEO) or low earth orbit (LEO), travel relative tothe Earth's rotation and typically provide high elevation angle coverageat the higher and lower latitudes, and since they are closer to earth,propagation time delays are minimized. Because of the unavailability ofstationary positions it is desirable to deploy NGSO satellites.

[0007] In one known implementation of a NGSO satellite system, severaldisadvantages are apparent. In the known system, each satellite isdeployed it its position individually. One drawback to individuallaunches is the high cost associated with each launch. Another drawbackis that the system is not easily adaptable to increasing demand.Individual launches must be used to provide increased coverage.

[0008] Data rates up to 19.2 kbps, as available from wireless WANs, willnot meet future data rate needs of consumers. For example, many computerusers are upgrading their wired modems to 56.6 kbps whenever possible.Such users desire a fast response from their modems even while they areaway from their desks. In addition, the nature of the information beingtransferred is changing from short, text-based electronic mail messagesto communications with embedded video clips. Such media-rich messagesconsume high bandwidth and communications resources, thus requiring highdata rates to allow them to be transmitted and received within areasonable period of time.

[0009] Furthermore, a tremendous growth in Internet traffic has caused astrain on the capacity of telephony networks. Network shortcomingsinclude network outages, insufficient access bandwidth, and insufficientinternode bandwidth. Currently, providers need to make significantinvestments, as well as experience installation delays, to upgradenetwork infrastructure, yet they cannot pass the costs on to the endusers.

[0010] Corporate LANs/WANs also generate an insatiable demand for higherbandwidth. The demand for bandwidth goes up as more and more users areconnected. The users, in turn, demand more services and improved networkspeed. Personal computers are being used to process not only text, butgraphics and video as well, all on networks that are increasinglyglobal. Widespread implementation of corporate intranets and extranetsfurther drive the move to increased bandwidth applications. High-speednetworking is also driven by the growth of video distribution,client/server technology, decentralized systems, increased processingpower and developments in storage capacity.

[0011] Fixed service demand such as satellite news broadcast, distancelearning, and military functions are continually increasing. It would bedesirable to provide a system capable of meeting demand of such uses.

[0012] Thus, there exists a need for a satellite communications systemthat provides communications to mobile users as well as fixed serviceusers. There also exists a need for a satellite communications systemthat provides global communications service while maximizing the usefulcapacity of the satellites, reducing the perceived time delay, andmaximizing the minimum elevation angle across latitudes.

DISCLOSURE OF THE INVENTION

[0013] The present invention provides a satellite communications systemwhich provides global network services to fixed and mobile users. Thesystem utilizes a first deployment of a plurality of satellites deployedin a medium earth orbit (MEO) and a few subsequent deployments of aplurality of satellites deployed in the same medium earth orbit (MEO) orother orbits. A ground terminal is provided for communicating with thefirst and the later deployments.

[0014] In one aspect of the invention, the satellites may be deployed inat 15000 km. One advantage of using 15000 km is that the satellitesavoid interference with the Van Allen radiation belts. Another advantageis that polar orbiting satellites need not be deployed.

[0015] One advantage of the invention is that a one dimensional trackingground antenna may be employed. A one dimensional tracking antenna isless expensive than two-dimensional antennas.

[0016] Another advantage of the invention is that the system isextremely adaptable in a business sense. That is, the system can bedeployed in a first configuration. Then, as the needs of the users ofthe system increase, further satellites may be deployed. The firstdeployment may be spaced to easily accommodate the second deployment sothat the second deployment may be accomplished in a single launch.

[0017] Another advantage of the invention is that the constellation ofthe present invention promotes frequency reuse. That is, because the MEOsatellites of the present invention are not in a direct line with GSOsatellites, the frequencies of GSO satellites may be reused in thepresent constellation.

[0018] Yet another advantage of the present invention is that some timewill lapse between the initial deployment and later deployments. Thus,the later deployments may take advantage of the newest technology, whichis important in the rapidly changing satellite technology industry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a diagrammatic representation illustrating a satellitecommunications system of the present invention.

[0020]FIG. 2 is a schematic illustration of a satellite of FIG. 1 viewedfrom the edge of the equatorial plane.

[0021]FIG. 3A is a schematic illustration of an initial operationconfiguration constellation of communications satellites utilized in thepresent invention.

[0022]FIG. 3B is a schematic illustration of a constellation ofcommunications satellites after a second deployment.

[0023]FIG. 3C is a schematic illustration of a constellation ofcommunications satellites after a third deployment.

[0024]FIG. 3D is a schematic illustration of a constellation ofcommunications satellites after a fourth deployment into an inclinedorbit.

[0025]FIG. 4 is a schematic illustration of satellite coverage usingspot beams.

[0026]FIG. 5 is a plot of latitude with 100% coverage versus number ofsatellites in the constellation at an elevation of 10000 km.

[0027]FIG. 6 is a global plot illustrating coverage and elevation anglein a constellation using 4 satellites.

[0028]FIG. 7 is a global plot illustrating coverage and elevation anglein a constellation using 8 satellites.

[0029]FIG. 8 is a global plot illustrating coverage and elevation anglein a constellation using 12 satellites.

[0030]FIG. 9 is a plot of latitude with 100% coverage versus the numberof satellites in the constellation at an altitude of 15000 km.

[0031]FIG. 10 is a graph of the capacity versus time plot illustratingthe advantages of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0032] Referring to FIG. 1, the communication system 10 with a typicalgeometry for practicing the invention is diagrammatically illustrated.In general, system 10 includes a plurality of communication satellites12 in middle earth orbit (MEO). As will be further described below,system 10 may also include satellites in an inclined medium earth orbit.The MEO satellites 12 provide quick introduction of regional servicesand low cost service over selected regions. The system 10 has a usersegment 14 and a ground segment 16. User segment 14 generally comprisesa number of fixed terrestrial sites 18 as well as a number of mobilesites. Mobile sites may include vehicles such as an airplane 20, atractor-trailer 22, and a ship 24. Various applications within thevehicles may incorporate satellite-based communication includingnavigation and communication applications.

[0033] Fixed sites 18 may be used as satellite operational centers fortracking and communication control, as network operational centers forvarious communications such as Internet connections, or beacon stationsfor satellite position control. Fixed sites 18 may, for example, becoupled to a terrestrial communications link 26. Terrestrialcommunications link 26 may, for example, be a connection into existingphone lines or cable/television lines.

[0034] User segment 14 and ground segment 16 each have an antenna 28.For ground segment 16, a fixed one-dimensional antenna 28 is preferred.One-dimensional tracking may be used due to the repeating path of themedium earth orbit satellites. For mobile communications, a twodimensional tracking antenna 28 is required. Antennas 28 are preferablyelectronically directed toward satellite 12 during movement. The systemmay communicate using V band or other suitable frequencies.

[0035] Various applications of a communication system formed accordingto the present invention include video conferencing, distance learning,corporate training, worldwide web games, internet-based interactiveservices, corporate communications, collaboration between suppliers andvendors, telemedicine and telehealth applications and satellite newsgathering. Particular industries that may benefit from such a satellitecommunication system would be industries that have internationalexposure and provide international services such as the airline,shipping, cruise, and businesses with some international exposure. Forexample, parcel delivery services, airlines, and cruise lines will beable to track assets on a global basis.

[0036] Referring now to FIG. 2, earth 30 has an equatorial plane 32 thatdivides the upper hemisphere and lower hemisphere. As shown in FIG. 2,an edge view of equatorial plane 32 is shown. One satellite 12 is shown,however, the constellation preferably comprises at least four satellitesin an initial operation configuration (IOC). The elevation angle 34 isshown from a point on the earth to satellite 12 with respect toequatorial plane 32. With four satellites in the IOC, semi-globalcoverage may be achieved. This means that most of the highly populatedareas of the globe may have service from the system. As will be furtherdiscussed below, by increasing the number of satellites in the system,ubiquitous coverage of the globe may be achieved.

[0037] Satellites 12 are preferably deployed in medium earth orbit at adistance of at least 10000 km from the Earth's surface. Satellites 12may be deployed between about 10000 km and 17000 km. In a preferredembodiment, satellites 12 are deployed at about 15000 km. By deployingthe satellites at 15000 km, elevation angles are increased in the mostpopulated latitudes of the earth.

[0038] For perspective purposes, a geostationary satellite 36 isillustrated. Geostationary satellites 36 are deployed at about 35000 km.This is over twice the distance of communication satellite 12 in mediumearth orbit. The path of satellites 12 is not in the line-of-sight ofGSO satellites 36, except for a narrow region around equator 32. Oneadvantage of the system is that frequency reuse may be accomplishedbetween the satellites 12 of the present invention and GSO satellites36.

[0039] Referring now to FIGS. 3A, communication satellites 12 areillustrated in a single plane in an initial operation configuration(IOC). Preferably, the satellites 12 are deployed in the equatorialplane. To provide semi-global coverage, four satellites are preferablydeployed. Because each satellite is equipped with position-adjustmentthrusters (not shown), a single launch vehicle may be used to deploy allfour satellites in a single launch. This significantly reduces the IOCcost.

[0040] Once in orbit, the positions of the satellites 12 may be adjustedby east/west station keeping. That is, satellites 12 may be temporarilyadjusted to a higher elevation (outward from earth), which slows themovement of the satellite. When the desired position is approaching inthe lower orbit, the satellite elevation can be moved inward to theproper position in the desired orbit.

[0041] After launch, the first deployment is shifted into place usingthe thrusters. The first deployment of satellites 12 have orbital voids39 therebetween.

[0042] Referring now to FIG. 3B, a satellite constellation in mediumearth orbit is illustrated after a second deployment. In this case, fouradditional satellites were deployed simultaneously in the seconddeployment. The second deployment is shifted into the orbital voids 39after they are placed into orbit. This second deployment, as will bedescribed further below, increases the elevation angle at the mostpopulated elevations. Satellites 12 may be positioned by east/weststation keeping. as described above so that the second deployment isinterleaved between the first deployment.

[0043] Referring now to FIG. 3C, if further satellites are deemed to berequired from the second deployment, a third deployment may be employed.In this case, four satellites are launched simultaneously to obtain thethird deployment. Prior to the third deployment, however, satellites 12may be shifted in their orbiting positions so that the single thirddeployment may deploy each of the last four satellites 12.

[0044] Referring now to FIG. 3D, if demand on the satelliteconstellation is increased further, more medium earth orbit satellitesmay be deployed on the equatorial plane. However, if spacing between theMEO satellites becomes too small, then satellites 12 may be deployed inan inclined orbit 38. The inclined orbit illustrated is also preferablyfilled with medium MEO satellites. The satellites 12 in the inclinedmedium earth orbit are also preferably launched by the single launchvehicle and are adjustable within their orbit paths. This will allowmore groups of satellites to be positioned in a single launch in theinclined orbit 38.

[0045] If the business demands on the system are such that furthersatellites are required, additional planes inclined at various angleswith respect to the equatorial plane may also be deployed. Also,various. numbers of satellites may be deployed within those inclinedorbits..

[0046] Referring now to FIG. 4, a preferred spot beam design isillustrated with respect to the Western Hemisphere. In thisillustration, a GSO satellite sharing belt 40 is illustrated in theequatorial region. The GSO satellite sharing belt 40 is plus or minus 11degrees of latitude. That is, the total belt north to south is 1224 km.To achieve the desired coverage, 253 beam positions are covered by thesatellite 50 beams maximum will be illuminated. The beams are referredto generally by the reference numeral 42. Preferably, each satellitecovers 90 degrees longitude. Thus, four satellites would providecoverage for most regions of the earth in the first phase. The beamsgenerated are preferably 2.5 degrees in width.

[0047] Referring now to FIG. 5, a plot of latitudes with 100 percentcoverage versus the number of satellites in a constellation at anelevation of 10000 km is illustrated with respect to various elevationangles. As is shown at point 44, at about 39 degrees latitude, 100percent of the area is covered by at least an elevation angle of 0degrees. At about 55 degrees latitude, 100 percent coverage is attainedif the elevation angle requirement is ten degrees using eight satellitesas illustrated at point 45. Coverage of 100 percent is achieved at a 10degree elevation angle at about 57° latitude with greater than twelvesatellites in the equatorial plane as illustrated at point 46.

[0048] Referring now to FIG. 6, a map of the world is illustrated withvarious elevation angles of coverage. The coverage for four satellites50 at an altitude of 10000 km on the equatorial plane is illustrated.The uppermost 52 and lowermost 54 regions of the map have no coverage.The next band lower 56 in latitudes has zero to ten degrees elevationangle coverage. The next band 58 in latitude has 10 to 20 degreescoverage in latitude, and the center portion 60 of the map has over the20 degrees elevation angle.

[0049] Referring now to FIG. 7, a similar map to that of FIG. 6 isillustrated. In this illustration, eight satellites 70 deployed at 10000km in altitude on the equatorial plane are shown. The same referencenumerals as in FIG. 6 are used to identify the various regions ofcoverage.

[0050] Referring now to FIG. 8, the coverage for 12 satellites 80 at analtitude of 10000 km on the equatorial plane is shown. As can be seen,most land regions of the earth are in areas having between. 0 and 10degrees of elevation angle. The same reference numerals as in FIG. 6 areused to identify the various regions of coverage.

[0051] Referring now to FIG. 9, a graph similar to that of FIG. 5 isshown with satellites having a 15000 km altitude. The advantage of usinga 15000 km altitude rather than a 10000 km is evident. At point 89,similar elevation angles as that shown in FIG. 5 are present at 50degrees latitude. For example, at latitudes near 60 degrees, elevationangles up to ten degrees may be achieved using eight satellites asillustrated at point 90. At point 91, about 630 latitude, 100 percentcoverage may be achieved with twelve satellites. Increasing thesatellite distance to 15000 km increases all of the elevation angles.

[0052] In operation, a first plurality of satellites is launched into amedium earth orbit in a first configuration. When traffic on thesatellites approaches capacity for communications, the second pluralityof satellites is deployed in medium earth orbit. The second plurality ofsatellites is preferably launched in a single launch. The first set ofsatellites and the second plurality of satellites are then interleavedin the same orbital path. Preferably, the satellites are in orbit 15000km above the earth. This allows higher elevation angles to higherlatitudes of the earth.

[0053] If further satellites are required because of increasingcommunications on the satellite, further deployment of satellites may belaunched in a similar manner. When the spacing approaches capacity forthe medium earth orbit, inclined medium earth orbits may be employed.Several planes of medium earth orbit satellites may be used tosubstantially increase the capacity of the system. Preferably, each ofthe deployments preferably is deployed in groups from a single launch.

[0054] Satellite technology is rapidly changing. The second deploymentis launched some time after the first deployment. This allows the seconddeployment and subsequent deployments the opportunity to use morecurrent technology. Also, because groups of satellites are deployedtogether, the launch schedule is more flexible than launching individualsatellites into their exact positions.

[0055] Referring now to FIG. 10, several advantages of the system asdescribed with respect to the present invention are illustrated. As canbe seen, actual demand may not align with projected demand for thesystem. In this manner, if actual demand is lower, a second orsubsequent launch may be delayed until demand increases. Likewise, ifdemand increases more rapidly than expected, the launch schedule can bemoved forward. The advantage of the lower cost of the initial systemmakes the system more practical than other systems where a full systemis initially deployed at a high cost.

[0056] While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

What is claimed is:
 1. A communications system comprising: a firstdeployment of a plurality of satellites deployed in a medium earth orbiton an equatorial plane; a second deployment of a plurality of satellitesdeployed in the medium earth orbit on the equatorial plane interleavedwith said first deployment.
 2. A communications system as recited inclaim 1 wherein said medium earth orbit is substantially about 15000 kmabove the earth.
 3. A communications system as recited in claim 1wherein said first deployment reuses the same. frequencies asgeostationary satellites.
 4. A communications system as recited in claim1 wherein said second deployment reuses the same frequencies asgeostationary satellites.
 5. A communications system as recited in claim1 further comprising a third deployment of satellites deployed on anorbit inclined with respect to the equatorial plane.
 6. A communicationssystem as recited in claim 5 wherein said third deployment reuses thesame frequencies as geostationary satellites.
 7. A satelliteconstellation as recited in claim 1 wherein said satellites communicateat c, ku or ka bands.
 8. A communication system for mobile and fixedservice users, comprising: at least four satellites in a medium earthorbit spaced in a first configuration to provide semi-global coverage.;at least one ground terminal having a fixed one-dimensional antenna; andat least one ground terminal having a two-dimensional tracking antenna.9. A communication system as recited in claim 8 wherein said at leastfour satellites are position-adjustable satellites that are spaced apartsuch that subsequently deployed satellites can be interleavedtherebetween.
 10. A communication system as recited in claim 8 whereinsaid at least one ground terminal provides tracking and communicationcontrol at a fixed site.
 11. A communication system as recited in claim8 wherein said at least one ground terminal provides network operationalcontrol for various communications.
 12. A communication system asrecited in claim 8 wherein said at least one ground terminal providessatellite position control at a fixed site.
 13. A communication systemas recited in claim 8 wherein said at least one ground terminal iscoupled to a terrestrial communications link.
 14. A communication systemas recited in claim 13 wherein the terrestrial communications link is aphone line.
 15. A communication system as recited in claim 13 whereinthe terrestrial communications link is a cable/television line.
 16. Acommunication system as recited in claim 9, further comprising a secondplurality of satellites interleaved between said at least foursatellites to increase the elevation angle at the most populatedelevations.
 17. A communication system as recited in claim 9, furthercomprising at least one additional satellite deployed in a medium earthorbit and positioned between two adjacent satellites of the at leastfour satellites.
 18. A communication system as recited in claim 17,further comprising additional satellites in an inclined medium earthorbit.
 19. A communication system as recited in claim 17, wherein saidadditional satellite uses more current technology than said at leastfour satellites.
 20. A communication system as recited in claim 8,wherein said at least four satellites are in the equatorial plane.